Water/lava can flow outside of the world border

The bug

Water placed within the bounds of the world border are capable of flowing out of it.

This was originally reported as MC-54772, which was resolved as a duplicate of MC-54119, which was fixed in 1.18 Pre-release 1, but this specific behaviour has still no been fixed.

How to reproduce

1. Go to the world border
2. Place a liquid source block which would flow outside of the world border

Expected results

As terrain outside of the world border is meant to be protected, the water/lava would not flow outside of the world border, either stopping or treating it as a solid block.

Actual results

It flows outside of the world border anyway.